An evaporating star may explain planets that orbit backwards.

The Solar System is a remarkably orderly place. The eight planets orbit in the same direction the Sun rotates and in nearly the same plane as the Sun's equator. Many smaller bodies—asteroids, comets, etc.—do the same. However, exoplanets don't always play as nicely, and a noticeable fraction even have orbits opposite their star's rotation. The widely accepted models of planet formation can't describe these misaligned exoplanetary systems.

A new idea proposed by Konstantin Batygin may help resolve the problem. In this revised model, strongly misaligned orbits are the result of another factor that influenced planet formation: a second star in the system. The gravitational influence of the companion star twisted the orbit of the exoplanet, pulling it out of alignment. And, in many cases, the star would leave little trace beyond the altered orbits: Sun-like stars often form in pairs or larger assemblies, but some of them evaporate over time.

Exoplanets are often detected using transits: when the planet passes in front of its host star, it produces a small eclipse. This is most prominently used by the Kepler observatory. In addition to indicating the presence of a planet, these transits create an unexpected optical illusion: they make the star look like it's moving (or, at least, moving more than it already is).

Because the star itself is rotating, one part of the star's disc will be moving toward us and one part will be moving away at all times (unless we happen to be looking straight down on one of the poles). The primary effect is to make star's emission spectrum appear broadened. During a transit, the planet blocks part of the light from one side of the star, which cuts off part of this indication of motion. When this happens, the star will appear to be moving toward or away from us when it actually isn't. This is known as the Rossiter-McLaughlin effect.

While the Rossiter-McLaughlin effect has not been observed in all transiting systems, in some cases astronomers can use it to measure the direction of the star's rotation relative to the direction of the planet's orbit. A substantial fraction of "hot Jupiters" (high-mass exoplanets with orbits smaller than Mercury's) have been observed to orbit opposite to their host star's rotation.

The most widely accepted models of planet formation have trouble explaining this misalignment. According to the best theories, planets form from a flat protoplanetary disk that was once contiguous with the material that formed the rapidly spinning new star, meaning they all orbit in the same plane and in the same direction as the star's rotation. Hot Jupiters migrate inward in this scenario, as they lose energy through friction with the gas remaining in the protoplanetary disk.

But hot Jupiters with highly misaligned orbits do not fit easily into this model, even though it does a reasonably good job of explaining our Solar System and many exoplanet systems.

The model proposed by Batygin modified the standard picture in a straightforward way. Ordinary single-star systems formed according to the widely accepted model. However, when a planet is formed in a binary system, the gravity of the companion star would have a profound effect on the protoplanetary disk when the two stars' spins were not aligned. Not only would some planets migrate inward to become hot Jupiters, the disk itself would tilt drastically in accordance with the total gravitation and spin of the two stars. In some cases, the disk could have flipped entirely over, leaving the hot Jupiter orbiting in the opposite direction to its star's rotation.

As many binary star systems don't persist, the companion star would often leave the system, with the only sign of its former presence being the highly misaligned exoplanet orbit. That does pose some problems for testing, as we have very few direct observations of protoplanetary disks. So, the chances of catching a binary system with a skewed disk are pretty slim.

Batygin suggested comparing the incidence of misaligned exoplanet systems to the known binary star formation statistics. If these numbers agree, it would be highly suggestive, if not sufficient by itself, to establish the model as the best possible alternative for explaining these strange hot Jupiter systems.

30 Reader Comments

The article's title and text don't appear to go together. The latter is describing a situation where a binary pair separates into two individual stars. While I think it's use is confusing, it's clear from the article that evaporate is referring to the breakup of the binary pairing; not to one of the stars dispersing into a cloud of gas.

Here I show that misaligned orbits can be a natural consequence of disk migration in binary systems whose orbital plane is uncorrelated with the spin axes of the individual stars. The gravitational torques arising from the dynamical evolution of idealized proto-planetary disks under perturbations from massive distant bodies act to misalign the orbital planes of the disks relative to the spin poles of their host stars.

The DOI abstract only deals with how the misalignment could occur in the first place, it makes no mention of what happens to the other star. Whether the article itself does, I cannot say.

Because the star itself is rotating, one part of the star's disc will be moving toward us and one part will be moving away at all times (unless we happen to be looking straight down on one of the poles). The primary effect is to make star's emission spectrum appear broadened.

Oh, wow. I didn't know stars turned fast enough to have detectable red-shift from one side to the other. That is pretty cool.

Quote:

However, when a planet is formed in a binary system, the gravity of the companion star would have a profound effect on the protoplanetary disk when the two stars' spins were not aligned.

I would have thought that binary star systems that formed from the same gas cloud would end up spinning the same direction. Is this just shifting the question?

As many binary star systems don't persist, the companion star would often leave the system,

Maybe I'm misinformed, but to the best of my knowledge the only interactions that can cause a body to be ejected involve 3 bodies. I'd guess that with 2 stars and a planet the planet could be ejected, but the planet (even a hot Jupiter) does not have the mass to perturb a star, much less cause an ejection. So the ejection of a star would require a 3 star interaction. If that were the case there would still be 2 stars remaining after the ejection.

As many binary star systems don't persist, the companion star would often leave the system,

Maybe I'm misinformed, but to the best of my knowledge the only interactions that can cause a body to be ejected involve 3 bodies. I'd guess that with 2 stars and a planet the planet could be ejected, but the planet (even a hot Jupiter) does not have the mass to perturb a star, much less cause an ejection. So the ejection of a star would require a 3 star interaction. If that were the case there would still be 2 stars remaining after the ejection.

Think of a binary star system merrily waltzing away when a third body does a fast fly-by. In that case, however, the third body would do a doozy on the planetary disk as well.

Do we observe these retrograde hot jupiters in the same systems as other normal planets? My hot-jupiter-companion idea would not predict the whole system running retrograde, but there could be different causes in diferent systems.

Do we observe these retrograde hot jupiters in the same systems as other normal planets?

Good question. I haven't kept up since I noted that they were no stumbling block to system formation models (as there were mechanisms tentatively suggested). At the time there were only solitary retrogrades, it was early days and the systems were crazy hot jupiters.

As many binary star systems don't persist, the companion star would often leave the system,

Maybe I'm misinformed, but to the best of my knowledge the only interactions that can cause a body to be ejected involve 3 bodies. I'd guess that with 2 stars and a planet the planet could be ejected, but the planet (even a hot Jupiter) does not have the mass to perturb a star, much less cause an ejection. So the ejection of a star would require a 3 star interaction. If that were the case there would still be 2 stars remaining after the ejection.

Think of a binary star system merrily waltzing away when a third body does a fast fly-by. In that case, however, the third body would do a doozy on the planetary disk as well.

That is one possible scenario, but I would think stellar near misses of this type extremely unlikely and I don't expect them to happen often enough to account for all the retrograde orbits detected. And as you say, there are other complications.

Oh, wow. I didn't know stars turned fast enough to have detectable red-shift from one side to the other. That is pretty cool.

Yeah, emission and absorption features in spectra are both very common and occur at such exact frequencies that it is quite easy to detect even VERY small differences in velocity, on the order of 2 m/s in fact. If we could isolate reflected light from a planet we would probably be able to determine its rate of rotation as well.

Quote:

Quote:

However, when a planet is formed in a binary system, the gravity of the companion star would have a profound effect on the protoplanetary disk when the two stars' spins were not aligned.

I would have thought that binary star systems that formed from the same gas cloud would end up spinning the same direction. Is this just shifting the question?

Maybe, and maybe not, presumably some sort of turbulence or other motion within a cloud results in fragmentation. In some cases it may be that different regions of the cloud end up spinning in different directions (IE at angles to one another. I think it is also true that even in fairly orderly clouds the motion only averages out. At first things are going in various directions, and only as the cloud collapses a more disk-like uniformly rotating structure evolves. Maybe in some cases multiple systems just don't quite get there.

This rogue planet is the kind of object I was thinking of when I said hot jupiters might be failed stars.

The refutable hypothesis is that the mutual orbit of the star and failed star would be no more related to the rest of the planetary disk than would any typical binary star (don't know the dynamics of planets in binary systems).

Ok smart guys -- now why does Venus rotate the opposite direction of all the other planets?

We're talking about revolution (travel around the star) as opposed to rotation (spinning on its own axis). Rotation can be perturbed by any number of things, and for a while it was thought to be tidally locked with Earth.

The evidence of large body (planetesimal) size impacts is quite compelling. Imagine then, that a body narrowly misses and gets inserted by the gravitational slingshot effect into a retrograde orbit. Seems to me that with the number of star systems, probably starting with on the order of 100 or so planetesimals, that retrograde orbits should NOT be uncommon.

The evidence of large body (planetesimal) size impacts is quite compelling. Imagine then, that a body narrowly misses and gets inserted by the gravitational slingshot effect into a retrograde orbit. Seems to me that with the number of star systems, probably starting with on the order of 100 or so planetesimals, that retrograde orbits should NOT be uncommon.

It is pretty hard to imagine a slingshot that would put a planet several times the mass of Jupiter into a retrograde orbit. I suspect its not energetically feasible, or at the very least that the 3rd body would have to be MUCH more massive. Even then I'm dubious you can transfer that much angular momentum in one shot. Maybe it can happen, but it sounds like one of those 'eye of a needle' kind of things.

Oh, wow. I didn't know stars turned fast enough to have detectable red-shift from one side to the other. That is pretty cool.

Yeah, emission and absorption features in spectra are both very common and occur at such exact frequencies that it is quite easy to detect even VERY small differences in velocity, on the order of 2 m/s in fact. If we could isolate reflected light from a planet we would probably be able to determine its rate of rotation as well.

It helps that the wavelength of light is something that is very easily measured with precision. One thing that impressed me in my university physics class (which I hope wasn't lost on other students) was that we were lucky to measure anything in that class to more than 2 or 3 significant figures. However, when we did the lab to measure the wavelength of a laser beam we managed 6 or 7 digits! We did this with crude instruments that you could have had in the 1700s. So it should be possible with modern instruments to detect incredibly small shifts.

AnonymousRich, an 'evaporated' companion star indeed implies a triple-star interaction, and yes that interaction also implies that two stars remain behind after the interaction that evaporates the wide-binary companion star, but the remaining (close) binary pair have likely long-since merged in a luminous red nova (LRN), leaving behind a solitary star with elevated metalicity due to nucleosynthesis in the LRN.

Multiple star systems form with interplay from which hierarchy emerges, causing a wide-binary system to spiral outward as one or more close-binary pairs spiral inward until they merge.

And an alternative conceptual hypothesis suggests that Proxima (Centauri) may be the wide-binary companion star to our Sun in a temporary unbound hyperbolic orbit around the passing star Alpha Centauri AB, and both wide-binary components may have formed from merged close-binary pairs: the central binary pair merging to form the Sun at 4,567 Ma and the companion binary pair merging to form Proxima at 542 Ma.

The alternative conceptual model also suggests that super-sized earth and larger planets form in pairs by gravitational collapse, also known as gravitational instability (GI), from accretion disks spiraling in to close-binary stellar pairs. GI occurs in 'Trojan' L4 or L5 Lagrangian points, 60 degrees ahead and behind the smaller B-star in close binary systems, which are the only stable locations where vortexes can form leading to gravitational collapse.

Then the 'Trojan planets' spiral outward, driven by the same core-collapse mechanism that raises the orbits of wide-binary companion stars. Trojan planets thus formed would be gravitationally bound to the larger A-star and unbound to the smaller B-star, resulting in orbital asymmetry leading to persistent core-collapse perturbation with the smaller B-star. In our own solar system, Venus-Earth, Jupiter-Saturn and Uranus-Neptune may be Trojan planetary pairings.

So by this alternative hypothesis, hot Jupiters in low orbits are expected rather than problematical. The conventional model for accretion of gas-giant planets beyond the snow line followed by their inward migration by a mysterious coupling with the protoplanetary accretion disk is resolved.

The alternative conceptual model also suggests that super-sized earth and larger planets form in pairs by gravitational collapse, also known as gravitational instability (GI), from accretion disks spiraling in to close-binary stellar pairs. GI occurs in 'Trojan' L4 or L5 Lagrangian points, 60 degrees ahead and behind the smaller B-star in close binary systems, which are the only stable locations where vortexes can form leading to gravitational collapse.

Thanks so kindly for your thoughtful reply. Either you're going to take me down, or at the very least, I'll owe you a citation.

And you've put severe constraints on the highly-speculative conceptual hypothesis. So now I have to concede that for stable libration orbits around the triangular L4/L5 points, the close-binary ratio of m1 is > 25 times m2 + m3, so Proxima (with a mass of about 1/8 of the Sun) apparently couldn't have been born at a Trojan point around a central binary pair, but what about a Jupiter-Saturn pair or smaller?

(The evidence for a wide-binary companion star, Proxima, which was itself composed of a close-binary pair involves two Types of Oort cloud comets and their differentiation into chemically-oxidized, Type I planetesimals that aqueously differentiate into gneiss (dome) cores with hydrothermal schist and carbonate-rock mantles and chemically-reduced, volatile-enriched, Type II planetesimals that differentiate into granite (pluton) cores with hydrothermal greenstone mantles, which is too involved to get into here.)

My contention for secular-perturbation (core-collapse) 'spiraling out' of Trojan planets accompanied by 'spiraling in' of a central close-binary stellar pair hinges on the persistence of gravitationally bound and unbound states.

Agreed: planets formed in a circumbinary protoplanetary disk are stable and not subject to core-collapse perturbation since the planets are gravitationally bound to both A- and B-stars of a central binary pair and thus in orbital symmetry with the barycenter. But Trojan planets formed in L4 or L5 Lagrangian points would be gravitationally bound only to the larger A-star and unbound to the B-star, creating orbital asymmetry and thus core-collapse perturbation that would persist indefinitely beyond attaining circumbinary status.

Another way to look at the orbital asymmetry is in light of a "gravity-assist maneuver" that spacecraft can use to increase their speed with respect to the Sun, which is the same way that Jupiter can throw comets and asteroids out of the inner solar system.

And finally, the formation of Trojan planets by gravitational instability doesn't require a stable, circumbinary protoplanetary disk, merely an accretion disk which spirals in toward a pair of close-binary stars.